How does a dipole antenna work

[ASidd]

How does it interact with the magnetic and electric fields of the radio waves while receiving them?

 

[mfb]

Radio waves are electromagnetic waves. Imagine a plane wave, polarized in some way. If you put a conductor parallel to the direction of the electric field, you get an electric field which changes direction with the frequency of the radio waves (or 2 times this frequency, if you just count direction switches). This electric field leads to an alternating current in the antenna, and therefore the potential at the ends of the antenna change - this can be detected by electronics.

Most radio waves are not plane waves and not polarized nicely, but usually you get some component of radiation which has the correct orientation for the antenna to pick up.

 

[ASidd]

This may sound stupid but then what happens to the magnetic component of the waves? Does it also interact with the antenna somehow to create electrical current.

 

[sophiecentaur]

A changing magnetic field will induce an emf in a wire (it needn't be a loop).

 

[webberfolds]

There are many ways of interpreting that line, please specify which interpretation is the correct one.

If you put a conductor parallel to the direction of the electric field

 

[mfb]

This thread is more than 1 month old.
I cannot see multiple ways to interpret that line, can you show me some?

 

[webberfolds]

Well the direction of the electric field is at a right angle to the direction the EM wave would be going. So parallel to that would be under or over the direction of the electric field? I'm pretty sure there's more ways of interpreting that but is that the correct one? I'm not very good at reading in general, but I want to make sure, thanks.

This thread is more than 1 month old.
I cannot see multiple ways to interpret that line, can you show me some?

 

[davenn]

Well the direction of the electric field is at a right angle to the direction the light would be going. ...

what light ??

we are talking about radio wave E-M field


Dave

 

[webberfolds]

When I wrote light I meant an EM wave, some animals may be able to see radio waves so I included them as light. I changed my previous post, it's more clear now that way.

what light ??

we are talking about radio wave E-M field


Dave

 

[davenn]

try the ARRL Radio Handbook
it gives a very good background to radio transmission and reception including antenna theory


cheers
Dave

 

[mfb]

What do you mean by "over" or "under" the electric field? In planar waves (or waves similar to them), the electric field is the same everywhere in the plane perpendicular to the propagation direction of the wave.

 

[webberfolds]

So where exactly is parallel? Nevermind about the over and under thing.

What do you mean by "over" or "under" the electric field? In planar waves (or waves similar to them), the electric field is the same everywhere in the plane perpendicular to the propagation direction of the wave.

 

[webberfolds]

Thanks!

try the ARRL Radio Handbook
it gives a very good background to radio transmission and reception including antenna theory


cheers
Dave

 

[mfb]

Parallel

As examples:
If the polarization of light is vertical, the electric field is vertical, and your antenna has to be vertical to pick up the signal (at least with some vertical component).
With a circular polarization, the electric field is vertical->horizontal->vertical->..., and your antenna direction does not matter unless it is orthogonal to the direction of propagation of the wave (or has some orthogonal component).

 

[webberfolds]

So if the antenna is vertical and the electric field is vertical and the photon moves by the antenna but is a bit too far left to hit it directly, could the magnetic field induce current in the antenna?

Parallel

As examples:
If the polarization of light is vertical, the electric field is vertical, and your antenna has to be vertical to pick up the signal (at least with some vertical component).
With a circular polarization, the electric field is vertical->horizontal->vertical->..., and your antenna direction does not matter unless it is orthogonal to the direction of propagation of the wave (or has some orthogonal component).

 

[davenn]

Even with the antenna and the electric field being at 90 degrees to each other there will still be some signal induced into the antenna ... just not very efficiently

In the field, in pratical situations, we see a 25 to 30 dB difference in signal strength when the polarisation is out by 90 deg

Dave

 

[webberfolds]

If the polarization of the signal is vertical and the photon meets the vertical antenna straight on, will that generate the most current? I hope this has to do with dipole antennas.

Even with the antenna and the electric field being at 90 degrees to each other there will still be some signal induced into the antenna ... just not very efficiently

In the field, in pratical situations, we see a 25 to 30 dB difference in signal strength when the polarisation is out by 90 deg

Dave

 

[bahamagreen]

A dipole is when the two conductor feed line is attached at the center between two separate long elements. This configuration is usually horizontal. but may be vertical, sometimes an inverted "V" shape... etc.
A true "vertical" antennae is distinguished by being configured vertically but with a single element, the feed line attaching one lead to the vertical element and the other to ground (usually a ground plane of radiating rods on the ground or similar, lots of designs).

The noteworthy behavior of a dipole is the relationship between current and voltage at different parts of the antennae... it alternates from having high voltage and low current at the distal ends of the elements while having low voltage and high current at the center, to having low voltage and high current at the ends and high voltage and low current at the center.

That is the basic mechanism in principle for both sending and receiving... how that relates to the emission and reception of EM is more complicated - ARRL books have great info and practical "theory", but may not get down to the level of a suitable "physics" answer.

 

[mfb]

So if the antenna is vertical and the electric field is vertical and the photon moves by the antenna but is a bit too far left to hit it directly, could the magnetic field induce current in the antenna?

I think you have something like that in mind. Forget it. Light does not work that way. That is not a 3-dimensional picture! Imagine those fields everywhere in the (z,y)-plane, and it gets better. While it is possible to emit directed electromagnetic waves, you cannot really "miss" an antenna in the way you imagine it.

 

[sophiecentaur]

If the polarization of the signal is vertical and the photon meets the vertical antenna straight on, will that generate the most current? I hope this has to do with dipole antennas.

Photons are not like little bullets and they don't "meet and antenna" in the conventional sense. It is really not helpful to look at it this way. How could you even start to consider how this single photon (dimensions / extent totally unspecified) will interact with a piece of metal, which consists of a distribution of charges all over it? Stick to waves if you want to understand most non-QM phenomena. There is nothing wrong with ( you are not making any compromises) using the wave approach.

 

[webberfolds]

When a radio photon goes through an antenna it induces current. (I wasn't thinking of photons like bullets except I've heard that they go in straight lines if 'undisturbed'.) If the EM wave was VP and the antenna was VP and the photon went through the antenna would that induce the most current? Or do I have a misconception? I'm such a beginner so I am not very confident about my understanding of it. Thanks for the help!

Photons are not like little bullets and they don't "meet and antenna" in the conventional sense. It is really not helpful to look at it this way. How could you even start to consider how this single photon (dimensions / extent totally unspecified) will interact with a piece of metal, which consists of a distribution of charges all over it? Stick to waves if you want to understand most non-QM phenomena. There is nothing wrong with ( you are not making any compromises) using the wave approach.

 

[jtbell]

When a radio photon goes through an antenna it induces current.

When a photon gets absorbed by an atom or (presumably) an antenna, it does something. When a photon "goes through" or "past" an atom or antenna, nothing happens. Photons interact destructively, in an "all or nothing" fashion.

 

[webberfolds]

The EM fields surrounding the photon induce the current, right? That's what I meant. Once again I'm a beginner and don't really know much about this subject. Thanks for the help!

When a photon gets absorbed by an atom or (presumably) an antenna, it does something. When a photon "goes through" or "past" an atom or antenna, nothing happens. Photons interact destructively, in an "all or nothing" fashion.

 

[webberfolds]

I was wondering if I had it wrong before, It seemed strange that way, I don't really know what I'm talking about. So where is/are the strongest part(s) of the EM field? Thanks for the post!

I think you have something like that in mind. Forget it. Light does not work that way. That is not a 3-dimensional picture! Imagine those fields everywhere in the (z,y)-plane, and it gets better. While it is possible to emit directed electromagnetic waves, you cannot really "miss" an antenna in the way you imagine it.

 

[sophiecentaur]

The EM fields surrounding the photon induce the current, right? That's what I meant. Once again I'm a beginner and don't really know much about this subject. Thanks for the help!

This is just not an accepted model. I can't think where you would have read it. To get into this subject you really have to ditch conventional ideas. Photons can be treated as particles under some circumstances - the appear to transfer momentum to a particle with mass, for instance - but when you say "surrounding the photon" you are implying that it has a size (for it to be surrounded). You can lay that one to rest for yourself if you ask yourself just what 'size' a photon could have. Is it related to the wavelength of the particular radiation? That would make it in the order of kilometers in extent for an LF radio signal. How would something of that size be expected to interact with a tiny whip antenna (10cm) that you could find on a hand held receiver? The 'size' thing just gets you going round and round in circles and leads you nowhere. Just stick to the universally accepted idea that a photon is an amount of energy which is transferred when EM energy is emitted or absorbed by a 'system'.
If you can find a single book in which an antenna's behaviour is explained or predicted using photons then I should be interested. Why do 'beginners' (self confessed - above ) obsess about using photons to explain straightforward classical phenomena? I'll tell you why. It's because schoolteachers (those who teach at elementary levels) know so little about the real message of QM that, when required to 'teach it', at a time which is way too early for kids to grasp, they teach the 'little bullets' model. Most people never manage to leave that model behind, unfortunately.
Stick to waves, fields, Volts and Current - that's a hard enough way to explain most electromagnetic phenomena.

 

[webberfolds]

Okay, I often don't know a correct way of explaining what I mean. I wasn't implying that's photons have a size but anyway. I think I understand that "universally accepted idea" about photons now, thanks!

Photons can be treated as particles under some circumstances - the appear to transfer momentum to a particle with mass, for instance - but when you say "surrounding the photon" you are implying that it has a size (for it to be surrounded). Just stick to the universally accepted idea that a photon is an amount of energy which is transferred when EM energy is emitted or absorbed by a 'system'.
Stick to waves, fields, Volts and Current - that's a hard enough way to explain most electromagnetic phenomena.

 

[davenn]

As far as an antenna goes here is where the Voltage and Current are peaking...

Dave

 

[webberfolds]

What does "peaks" mean here, thanks for helping answer more about dipoles and congratulations on making it to 1000 posts! I realize now that some of my posts on this thread don't have much to do with dipoles, sorry, how do I fix that? I don't want to be rude and seem selfish.

As far as an antenna goes here is where the Voltage and Current are peaking...

Dave

 

[bahamagreen]

That picture is a single snapshot... the peaking of voltage and current alternates between:

high V low I at the ends with low V and high I at the center,

and

low V high I at the ends with high V and low I at the center.

 

[davenn]

What does "peaks" mean here, thanks for helping answer more about dipoles and congratulations on making it to 1000 posts! I realize now that some of my posts on this thread don't have much to do with dipoles, sorry, how do I fix that? I don't want to be rude and seem selfish.

Thanks for the congrats :) don't worry about your earlier posts, you were just getting tied up with and side tracked with looking at things from an individual photon point of view --- hopefully sophiecentaur's last post got you free of that and onto the right track :)

its all ok just continue to learn from what others and myself explain to you.
Hey I don't claim to be an expert either ! there are a good bunch of guys on this forum that are way above me in theoretical understanding.
Most of my understanding comes from practical applications and experiments I have done over many years. And I too learn from what those guys post :)

OK, the diagram is showing you where on the halfwave dipole antenna that the current and the Voltage peak. You can see that over a half wave of a cycle of the applied AC signal that the current (I) peaks at each end of the dipole and is at zero in the middle - at the feedpoint. Whereas the Voltage is opposite, it is at zero at the ends of the antenna and it peaks at the centre where the feedpoint is.

cheers
Dave